A typical form of apparatus for measuring unbalance on a rotary member such as a motor vehicle wheel comprises a main shaft on which the rotary member to be balanced is clamped by means of a suitable clamping arrangement. A drive motor is provided to drive the main shaft in rotation, by way of a reduction transmission disposed between the main shaft and the motor. The apparatus also includes a restraining means for holding the main shaft when the clamping arrangement is being actuated, that is to say tightened or loosened, for fitting a rotary member on to the main shaft or for removing a rotary member therefrom. The restraining means thus holds the main shaft to resist the torque which is applied thereto in the operations of tightening or loosening the clamping arrangement for fixing the rotary member on the rotary shaft. One form of such restraining means comprises a holding brake structure, for example in the form of an electromagnetically operated brake, an example of which is to be found in German laid-open application (DE-OS) No 37 30 084. Such apparatuses are to be found for example in balancing machines in a workshop for the balancing of motor vehicle wheels.
Furthermore, after a measuring run for detecting unbalance of a rotary member has been completed, for slowing the rotary member down to bring it to a stop, when the drive motor is in the form of a three-phase motor, the rotary field of the drive motor may be reversed to produce the deceleration effect. For the purposes of positioning the rotary member on which unbalance has been measured, to put it into the appropriate position for balancing thereof, use is made of a pulsating direct current which is supplied to the windings of the three-phase drive motor.
In that situation however, particularly when decelerating the rotary member to bring it to a halt after a measuring run has been carried out thereon, undesirable braking moment peaks may occur, which result in the rotary member being turned on the main shaft as the rotary member is fixed on the main shaft in the appropriate angular relationship therewith by the clamping arrangement, by means of a frictional engagement effect. If however, undesirable braking moment peaks result in the rotary member being turned relative to the main shaft, it is no longer possible to provide for a precise association as between the angular position of the detected and measured unbalance and a balancing position in which unbalance of the rotary member is to be removed, for example by fitting a balancing weight to the rotary member.